I know that most of the logs ( if not all ) are in the folder :

"YOURPATHTOSTEAM"\steam\SteamApps\common\The Divinity Engine\

There's osiris logs, dialog logs, error logs etc...

If the Log() function doesn't work, you might want to try your luck with other functions.
http://www.divinityengine.net/index.php?title=List_of_scripting_commands

DebugText (CHARACTER|ITEM target, STRING text)
Adds debugtext above the character/item for a short amount of time.

If all these don't work, you can send a message to osiris with any of the Event system.
ex:
GlobalSetEvent (STRING eventName)
Sets a global event. Scripts and Story can catch it.

In Osiris :

IF
GlobalEventSet("debugText")
THEN
DebugBreak("itworks!");